The present invention relates to a control device for a variable valve timing mechanism of an engine which changes the opening valve timing (valve timing) of at least one of an intake valve and an exhaust valve of the engine depending on an operation state of the engine. The invention more specifically relates to a control device for a variable valve timing mechanism of an engine which stops the operation of the variable valve timing mechanism to restrain a shock caused by a torque fluctuation in at least one of a case when an automatic transmission is in the process of transmission operation and a case when a lock up clutch is in the process of switching.
In recent years, engines with a variable valve timing mechanism which changes the valve timing of at least one of the intake valve and the exhaust valve in the engine depending on the engine operation state have been widely used. Such an engine controls as required the valve overlap period during which both valves are open at the same time, so that a combustion form suitable for the engine operation state can be implemented and the combustion efficiency can be improved.
Meanwhile, conventionally, it has been widely practiced to provide a vehicle having an automatic transmission with a so-called lock up clutch which engages/disengages an impeller and a turbine with/from a torque converter as required and thus mechanically couple the input side and the output side of the automatic transmission. Thus, the transmission loss in the torque converter is restricted to efficiently transmit driving force, so that the fuel efficiency is improved.
The engine with a variable valve timing mechanism sometimes employs the manner of control in which the valve timing is angularly delayed before cutting fuel, and vibration caused by the fuel cutting at the time of slow down is restrained. However, such control causes an abrupt output torque fluctuation, and vibration is caused through the body of the vehicle. Therefore, Japanese Patent Laid-Open Publication No. Hei. 8-246938 discloses cooperative control between a variable valve timing mechanism and an automatic transmission. According to the disclosed control, in the slow down state without the necessity of fuel supply, the lock up clutch is engaged followed by angular delay control and then fuel supply is cut, in order to restrain the vibration.
According to the structure disclosed by Japanese Patent Laid-Open Publication No. Hei. 8-232693, when the speed is changed to a low speed step by kick down for example, and the engine speed abruptly increases, the revolution number is previously estimated based on a target transmission ratio in the automatic transmission and the vehicle speed. Then, the advance angle value is corrected as required so that an appropriate valve lift characteristic for the estimated revolution number is attained. Then, the improper operation of the intake/exhaust valves may be prevented and therefore excessive allowance is not provided in a steady state, so that a valve lift appropriate for a driving condition may be provided.
Meanwhile, in a vehicle having an engine with such a variable valve timing mechanism, if there is a change in the valve timing during the transmission operation of the automatic transmission, the output torque fluctuation is so large that smooth transmission operation is impaired and a great transmission shock could result. More specifically, since the engine speed or load changes at the time of transmission, the target valve timing also changes accordingly. Thus, the synergistic effect between the transmission operation and valve timing switching increases the shock at the time of transmission, and therefore there is a demand for improvement to such a state in order to provide more comfortable driving conditions.
In this case, while the above publication describes techniques of alleviating the output torque fluctuation, they are all related to alleviation in the torque fluctuation at the time of shift down, and there is no technique disclosed which is particularly directed to alleviation in a transmission fluctuation at the time of shift up. More specifically, at the time of shift up by the automatic transmission, a change in the advance angle value by the variable valve timing mechanism could cause an abrupt torque fluctuation as a lock up clutch is engaged and the transmission shock could be great.
The present invention is directed to a solution to the above-described problem, and it is an object of the present invention to provide a control device for a variable valve timing mechanism of an engine which can prevent a shock caused by an abrupt torque fluctuation during transmission operation or the switching of a lock up clutch in the engine.
In order to achieve the above-described object, a control device for a variable valve timing mechanism of an engine installed in a vehicle having an automatic transmission according to a first aspect of the present invention includes valve timing control means for controlling an operation of the variable valve timing mechanism based on an engine operation state, and stopping the operation of the variable valve timing mechanism when the automatic transmission is in the process of transmission operation.
According to a second aspect of the present invention, in the first aspect of the invention, a torque converter having a lock up clutch capable of mechanically connecting an input side and an output side of the automatic transmission is provided between the engine and the automatic transmission, and the valve timing control means stops the operation of the variable valve timing mechanism in at least one of a case when the automatic transmission is in the process of transmission operation and a case when the lock up clutch is in the process of switching.
According to a third aspect of the present invention, in the second aspect of the invention, there is further provided transmission control means for controlling the automatic transmission and the lock up clutch based on the operation state of the engine and the vehicle, and the valve timing control means is provided with data fed from the transmission control means and stops the operation of the variable valve timing mechanism for a predetermined time period after transmission control data or lock up clutch control data is switched.
According to a fourth aspect of the present invention, in the first to third aspects, the valve timing control means prohibits a target valve timing set based on the engine operation state from being updated, and stops the operation of the variable valve timing mechanism.
More specifically, according to the first aspect of the present invention, during the transmission operation of the automatic transmission, the operation of the variable valve timing mechanism is stopped. Thus, the valve timing can be prevented from being changed because of a temporary fluctuation in the engine speed and in the engine load in association with the transmission operation, and an engine output fluctuation caused by an unnecessary fluctuation in the valve timing can be prevented, so that the torque shock can be restrained and the controllability improves.
The second aspect of the present invention is adapted to the case where a torque converter having a lock up clutch capable of mechanically connecting an input side and an output side of the automatic transmission is provided between the engine and the automatic transmission, and herein the operation of the variable valve timing mechanism is stopped when the automatic transmission is in the process of transmission operation or when the lock up clutch is in the process of switching. Thus, without mentioning the period during the transmission operation of the automatic transmission, the valve timing can be prevented from being changed in association with a temporary fluctuation in the engine speed and the engine load caused by the switching of the lock up clutch. As a result, the engine output fluctuation in association with an unnecessary change in the valve timing can be prevented and the torque shock can be restrained.
According to the third aspect of the present invention, data from the transmission control means controlling the automatic transmission and the lock up clutch based on the operation state of the engine and the vehicle is input and the operation of the variable valve timing mechanism is stopped for a predetermined time period after transmission control data or lock up clutch control data is switched. Thus, a response delay by a clutch or a brake in the automatic transmission or a response delay by the lock up clutch switching is compensated and the torque shock is more surely restrained to improve the controllability.
Furthermore, according to the fourth aspect of the present invention, the operation of the variable valve timing mechanism is stopped by preventing the target valve timing set based on the engine operation state from being updated, and holding the target valve timing at the time. Therefore, this control can be very readily incorporated in the conventional control.